1. Field of the Invention
The present invention relates generally to a R-T-B-M-C sintered magnet.
2. Description of the Prior Art
Since the invention of the sintered Nd—Fe—B permanent magnet by Mr. Sagawa and others in 1983, its field of application has been expanding continuously. Currently, the field of application includes initial medical magnetic resonance imaging (MRI), hard disk drives voice coil motor (VCM), CD Pickup Mechanism, medical and information technology. The field of application is also gradually expanding to include energy conservation and environmental protection fields such as new energy vehicles, generators, wind generators, air conditioning and refrigerator compressors.
Due to the increasing use of the sintered Nd—Fe—B permanent magnetic materials, rare earth material resources become scarce. Accordingly, there is an increasing need for improvements in the utilization of the rare earth materials. The traditional processing method of the rare earth materials includes using a steel molding process, suppressing the rare earth materials in a first direction and orienting the rare earth materials by applying a magnetic field that is perpendicular to the first direction to produce a compact. After suppressing the rare earth materials, the compact is subjected to an isostatic pressing process. Next, the compact is sintered and subjected to a heat treatment. Using the traditional processing method, it is very difficult to manufacture a compact having small dimensions from the rare earth materials due to mold size and other limitations. For example, using the traditional processing method, it is especially difficult to manufacture a compact from the rare earth materials having an orientation direction larger than 20 mm. With regard to manufacturing permanent magnets from the rare earth materials having a thin orientation direction, additional slicing and grinding are needed which will result in a loss of the rare earth materials. For example, in order to make small permanent magnets having a thickness of 3 mm, slicing process alone will result in a 10% loss in rare earth materials.
In order to improve the utilization of the rare earth magnetic materials, a parallel magnetic suppression process is developed. Using the parallel magnetic suppression process, orientation of the magnetic field and suppression of the rare earth magnetic materials are applied in directions parallel to one another. Accordingly, the permanent magnets can be formed without isostatic pressing and can be directly sintered and subjected to the heat treatment. Because the orientation of the magnetic field and suppression of the rare earth magnetic materials are applied in a direction parallel to one another, thin magnets can be directly formed. After directly forming the thin magnets, the thin magnets can be directly sintered and subjected to a heat treatment. By using the parallel magnetic suppression process, a higher utilization of the rare earth magnetic materials can be achieved because the permanent magnets can be manufactured and grinded without the slicing process. However, using the parallel magnetic suppression process can have a detrimental effect on the physical properties of the permanent magnets. For example, the parallel magnetic suppression process can affect the orientation degree of the permanent magnet, decrease magnetic remanence of the permanent magnet by 0.06-0.07 T, and reduce the magnetic energy of the permanent magnet by 10%.
Another method developed to improve the utilization of the rare earth magnetic materials is a non-pressure molding process. The first step of the non-pressure molding process is filling a mold with magnetic powders and orienting the magnetic powders in the mold by subjecting the magnetic powders to a magnetic field. After orienting the magnetic powders, the magnetic powders are sintered and subjected to a heat treatment. The orienting process is performed without applying pressure to the magnetic powders in the mold. In addition, heat can be introduced to the magnetic powders either before and/or after the orientation process. By adding heat to the magnetic powders, the coercivity of the magnetic powders is lowered, and the degree of orientation of the magnetic powders is increased. After the orienting process, the magnetic powders in the mold are sintered and subjected to the heat treatment. By using the non-pressure molding process, a higher utilization of the rare earth magnetic materials can be achieved because the permanent magnets can be manufactured and grinded without the slicing process.
There are also drawbacks associated with using the non-pressure molding process which will affect the physical properties of the permanent magnetic. The first drawback associated with the non-pressure molding process is that there is a decrease in the density of the magnetic powders. Since pressure is not applied to the magnetic powders during the orientation process, there is a repulsion force between the individual magnetic powder particles in the magnetic powders which lowers the density of the powder and the density of a sintered block obtained from the sintered process. The second drawback associated with the non-pressure molding process is that the magnetic powders are subjected to oxidation. Since the individual magnetic powder particles have a small particle size and heat is applied to the magnetic powders prior to and after the orientation process, in the presence of oxygen, the magnetic powders are prone to oxidation.